3-Chloro-N,N-dimethylpropan-1-aminium chloride

The organic cation in the title molecular salt is found with a gauche arrangement for the terminal C—C—C—Cl grouping, which DFT calculations show is the stable conformation relative to anti.


Structure description
The molecular structure of the title compound, C 5 H 13 NCl + ÁCl À , Fig. 1, corresponds to expected values with an average C-C bond length of 1.497 (8), an average C-N bond length of 1.482 (6) and a C-Cl bond length of 1.781 (7) Å . The bond angles for the sp 3 hybridized centers range from 108.7 (4) to 113.5 (4) . The Cl atom appears in a gauche conformation, with a Cl1-C1-C2-C3 torsion angle of À68.6 (6) , rather than in the anti conformation. The structure of the chloroethyl analog (Muller et al., 2021;CSD refcode: URORUR) shows an anti conformation for the chloro group (and a disordered alkyl chain) in a lower symmetry space group than the title compound (monoclinic I2/a). We were curious if the gauche conformation was a consequence of packing in the tetragonal space group or a property of the isolated molecule, and pursued a complementary computational study.
A DFT geometry optimization [B3LYP,6311+G(d,p); GAMESS (Schmidt et al., 1993)] in vacuo of the gauche conformation similar to that found in the title structure yields a torsion angle of À63.1 and a C-Cl bond length of 1.812 Å , while geometry optimization of the other gauche position yields a torsion angle and bond length of 64.5 and 1.813 Å , data reports respectively, with a slightly lower energy (by 0.0101 eV). In contrast, geometry optimization for the anti conformation yields a shorter C-Cl bond length (1.801 Å ) and a higher energy (by 0.0944 eV). For the chloroethyl analog, the gauche conformations are also more stable (by 0.226 eV) than the anti with a similar C-Cl bond lengthening (1.811 Å versus 1.795 Å ). These results are consistent with hyperconjugation, which places a -H atom in an anti-periplanar arrangement with Cl, i.e. the gauche effect. This anti-periplanar arrangement allows the back donation of the C-H bond electrons to the anti-bonding molecular orbital of the C-Cl bond with resulting C-Cl bond lengthening (Wolfe, 1972;Rodrigues Silva et al., 2021). Furthermore, the gauche conformation also places the partially negative Cl atom and formally positive N atom in proximity to enhance stability, as shown in the electrostatic potential plot of Fig. 2. This agrees with calculated ClÁ Á ÁN distances of 4.60 Å [gauche, 4.638 (4) Å , experimental] versus 5.27 Å (anti) for the title compound and, likewise, 3.07 versus 4.10 Å for the chloroethyl analog. With the greater calculated stabilization of the gauche conformation in the chloroethyl analog, it is surprising to see the anti conformation in URORUR. It is worth noting, though, that a gauche conformation is found for this cation in the hexachlorodioxodimolybdate(V) salt (POSWAX) with an ordered alkyl chain (Marchetti et al., 2015).
The extended structure of the title compound can be envisioned as layers of ion-pair formula units lying parallel to ab, shown in Fig. 3, with the structure built up by offset stacking of these layers along c due to the I centering translation. Within the layer, two motifs catch the eye as representative of 4 symmetry. One is a pinwheel structure in which the ends of the propyl chains of four organic cations meet at the center. Chloro groups at the center are directed above or below the layer plane with alternating orientations as one progresses around the pinwheel (Fig. 4). The other motif is a Electrostatic potential plot of the molecular cation in the title compound from the reported DFT calculation. Red represents the most negatively charged regions and blue the most positively charged regions.  Ball-and-stick diagram of a portion of the layer in the ab plane. The threedimensional structure is generated by offset stacking of these layers in the c-axis direction.

Figure 4
Ball-and-stick diagram of the pinwheel structural motif found in the layer depicted in Fig. 3. square with a formula unit on each edge in a head-to-tail arrangement with the chloride ion close to the ammonium head group on each edge (Fig. 5). The head-to-tail arrangement circulates in a counterclockwise direction looking down c. Application of a twofold rotation perpendicular to c generates the other twin component in which the sense of circulation is reversed. The square motif contains a void in the center about which the chloro groups from pinwheel motifs of neighboring layers, a pair from each arranged in a distorted tetrahedron, fit. The H atom of the ammonium group has the opposite orientation to the chloro group and hydrogen bonds to a chloride ion of the other neighbor layer (Table 1). Thus the only classical hydrogen bonding is interlayer. A packing diagram with unit cell axes is shown in Fig. 6.

Refinement
All non-H atoms were found during initial structure solution and refined anisotropically. A check using the PLATON routine TwinRotMat (Spek, 2020) suggested merohedral twinning about a twofold axis in the higher symmetry tetragonal point group 42m. Refinement of the twin model [BASF = 0.358 (2) for the minor component] resulted in a substantial drop in R-factor values, rectification of highly anomalous displacement ellipsoids, and the appearance of H atoms in the electron-density difference map. Crystal data, data collection and structure refinement details are summarized in Table 2.

Figure 5
Ball-and-stick diagram of the square structural motif found in the layer depicted in Fig. 3.

Figure 6
Capped-stick packing diagram for the title compound showing the sequential offset stacking of three layers and the interlayer hydrogen bonding that connects neighboring layers.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 2-component twin.